The present invention relates to a scroll-type hydraulic machine.
Before describing the present invention, the basic principles of a scroll-type hydraulic machine will be briefly explained.
FIGS. 1A to 1D show fundamental components of a scroll-type compressor, which is one application of a scroll-type hydraulic machine, and the operations thereof in successive angular positions. In these figures, the compressor is composed of a stationary scroll 1, having a fixed center O, and an orbiting scroll 2, which performs an orbiting motion around a fixed point O'. Compression chambers 4 are formed between the stationary scroll 1 and the orbiting scroll 2, and a discharge port 3 is formed around a center portion of the stationary scroll 1. The scrolls 1 and 2 take the form of spiral arms, each of which may be in the form of an involute or a combination of involutes and arcs. The arms are complementary in shape. The stationary scroll 1 and the orbiting scroll 2 are interleaved as shown.
In operation, the orbiting scroll 2 orbits continuously with respect to the stationary scroll 1 from a starting position (0.degree.) shown in FIG. 1A through operating cycle phase positions of 90.degree. (FIG. 1B), 180.degree. (FIG. 1C) and 270.degree. (FIG. 1D), without changing its angular orientation with respect to the stationary scroll 1. With such orbital movement of the orbiting scroll 2, the volumes of the compression chambers 4 are cyclically reduced, and thus fluid introduced therein is compressed. The compressed fluid is finally discharged from the discharge port 3. During this operation the distance between the center O and the fixed point O', which is maintained constant, can be represented by: ##EQU1## where p corresponds to a distance between wraps and t is the wall thickness of each wrap.
In order to minimize the thrust force of a scroll-type hydraulic machine or compressor having a large capacity, a structure has been proposed in which the orbiting scrolls are arranged in a back-to-back relationship to cancel out the thrust forces. Examples of such structures are disclosed in U.S. Pat. Nos. 801,182, 3,011,694 and 4,192,152. In order to facilitate an understanding of the background of the present invention, the structure having the back-to-back arranged orbiting scrolls will be described briefly with reference to FIG. 2, which shows schematically an example of such a structure as disclosed in U.S. Pat. No. 4,192,152.
In FIG. 2, a pair of stationary scrolls 1 have complementary-shaped wraps 5. The scrolls 1 are fixedly secured to each other by bolts 4 with the scroll wraps facing one another with a space therebetween. An orbiting scroll 2 is provided on opposite surfaces of a center plate with complementary-shaped orbiting scroll wraps 6. The orbiting scroll 2 is disposed in the space between the stationary scrolls forming a plurality of compression chambers 4 between the stationary scroll wraps 5 and the orbiting scroll wraps 6. Discharge ports 3 for the compressed fluid are formed at center portions of the stationary scrolls 1 to which respective discharge tubes 15 are connected. An intake port 16 is formed at a suitable peripheral position of one of the stationary scrolls 1 to which an intake pipe 17 is connected. Near the intake port 16 in the space between the stationary scrolls 1 is formed a suction chamber 18. A crankshaft 7 having an eccentric portion is supported by bearings 9, 10 and 11 provided in the stationary scrolls 1 and is driven through a coupling 12 by a drive source 13. The eccentric portion of the crankshaft 7 is supported by a bearing 8 provided in the orbiting scroll 2. A balance weight 19 is attached to the eccentric portion of the crankshaft 7 to balance a centrifugal force acting on the orbiting scroll 2 during the operation of the machine.
In operation, the crankshaft 7 is rotated by the drive source 13, which may be electric motor, internal combustion engine, turbine or the like. Upon the rotation of the crankshaft 7, an orbiting force is imparted to the orbiting scroll 2 via the bearing 8 by the eccentric rotation of the eccentric portion of the crankshaft. Compression then occurs on both sides of the orbiting scroll as described above. The pressure in the compression chambers 4 increases as the chambers 4 move towards the center portion of the machine and pressurized fluid is discharged through the discharge ports 3 and hence through the discharge tubes 15. At the same time, fluid intake occurs through the suction tube 17 and the intake port 16 to the intake chamber 18, which feeds the fluid to the compression chamber 4. The centrifugal force acting on the orbiting scroll 2 which is generated during the operation thereof is statically as well as dynamically balanced by the balance weight 19 shown in FIG. 2.
Since the compression chambers 4 are formed symmetrically around the orbiting scroll 2, the pressure distribution of the compression chambers 4 on both sides of the orbiting scroll 2 are similar, and thus there are no thrust forces acting on the orbiting scroll 2 as a whole. This construction is particularly effective when the operating speed of the orbiting scroll is low and the thrust load is large because, in such a case, it is very difficult to use a thrust bearing.
Although this conventional structure is advantageous due to the fact that no thrust force is produced, there are still problems in actual practice. Specifically, it is impossible as a practical matter to manufacture the orbiting scroll 2 having the complementary scroll wraps 6 on the opposite sides thereof with a high precision, and it is very difficult to assemble the orbiting scroll with the stationary scroll 1 with precisely controlled radial gaps between the orbiting scroll wraps 6 and the stationary scroll wraps 5 on both sides of the orbiting scroll. Particularly, the relative position of one stationary scroll to the other is determined by the relative positions of the bearings mounted in the stationary scrolls 1, and the relative position of the orbiting scroll 2 to the stationary scrolls 1 is determined by the coupling provided by the crankshaft 7. Thus, very precise adjustment of the radial gaps between the orbiting scroll and the stationary scrolls is impossible as a practical matter. Once these factors are taken into account, the conventional scroll-type machine constructed as described above has not been entirely satisfactory.
Another important problem relates to the driving system for the orbiting scroll. In FIG. 2, a single crank mechanism is used. In a case where a plurality of crank mechanisms are arranged equiangularly, the eccentric centers of the respective crankshafts 7 of the mechanisms must be highly precisely determined, otherwise a normal operation of the machine itself cannot be expected.
A more important problem resides in that, due to the fact that the drive system is disposed at the periphery of the orbiting scroll 2, the diameter of the orbiting scroll 2 is necessarily large, and due to a large mass resulting from such a large diameter of the orbiting scroll, the bearing load due to centrifugal forces is not negligible. Furthermore, the diameter of the stationary scrolls 1 is necessarily also large, which makes it necessary to make the walls of the stationary scrolls quite thick.